I. Field of the Invention
The present invention relates to a releasable holding mechanism with means for retaining the mechanism in a locked condition and to a method for actuating the mechanism, and is concerned particularly, although not exclusively, with a releasable holding mechanism for use in automotive roll-over protection systems (ROPS), releasable headrest, backrest, raisable hood for pedestrian protection and other automotive applications.
The invention is designed especially for ROPS, but it may be applied otherwise where there is a need for releasing a secured load, e.g. safety systems at sea, life rafts (as an alternative to rope-cutter devices), securing and releasing cargo, and release mechanisms for aero/space, etc.
II. Description of the Related Art
The Roll Over Protection System (ROPS) is installed in a vehicle to protect the driver and passengers if the vehicle should roll over. During normal driving the roll over bar or the protection member is stored in a lower, retracted position. The roll bar is only released and raised to an upper position when a roll over situation occurs and a signal is received from one or several sensors in the vehicle.
Known roll bars are normally sprung towards the upper position, but are held back in the lower position by a releasable holding mechanism, holding the roll bar via one or more connecting elements. Thus the holding mechanism in a locked position will keep the roll bar in the lower position and the holding mechanism in a released position allows the roll bar to be raised to the upper position.
The ROPS may comprise one or more separate roll bars or protection members using one holding mechanism for each. Several protection members may also be connected together using one holding mechanism. The holding mechanism to be used in a roll over protection system faces several problems:                Rough environmental conditions such as vibrations, tilting, dust and moisture that can cause the roll bars to either fasten in a lower position or release unintentionally.        Tolerances and deflections in the vehicle chassis create positioning problems and might cause the roll bar to release unintentionally or fasten.        The power and energy consumption of the release mechanism should be very low to avoid the need for extra power supplies like electrical capacitors. Ideally the mechanism could be powered directly by the crash control unit.        The holding mechanism should be regenerative. That means that the roll bar connection element should snap into locked position again after being released when it is pushed\pulled down manually or by another force.        
According to relevant prior art, releasable holding mechanisms to be used in a roll over protecting system are either activated by electromechanical or pyrotechnical devices. The roll bar is kept in its lower position by a mechanical catch holding element until a signal is given and the roll bar is released. Such mechanisms rely on an active release arrangement whereby some form of actuator is energized in order to move a piston or rod element to unlock the catch holding element which in turn releases the roll bar for deployment.
The pyrotechnical actuators are based on the principle that a small amount of propellant is ignited, and the pressure energy is used to disconnect the locking mechanism. The main problem is that they are not regenerative and after being used they must be replaced.
The electromechanical actuators such as the one disclosed in DE 100 02 723 are known to have a connection rod between the roll bar and the actuator. This connection rod is more or less directly connected to a release solenoid using e.g. a ramp (cone) and ball(s) to lock the roll bar in the lower position. This solution is disadvantageous in that a minor tilting of the connection rod leads to movement directly in the release mechanisms locking balls. As a result the roll bar may be released unintentionally. The mechanism is also partly open for pollution. The direct connection between the connection rod and the release mechanism requires high power and energy consumption when the actuator releases, because the actuator must work against most of the spring load.
Another example of a release mechanism comprising a solenoid actuator is a release mechanism as described in the patent specification U.S. Pat. No. 5,626,361, in the name Bayerische Motoren Werke AG.
In U.S. Pat. No. 5,228,730 there is disclosed an apparatus for converting mechanical locks to operate electrically using momentary power. A lock knob can be arrested by means of a locking pin. The movement of the locking pin is generated by means of a combined electromagnet and a permanent magnet together with a spring. By alternating the polarity of the power, the position of the locking pin can be alternated between a locked position and an unlocked position. This solution does not relate to hold and release of a pre-tensioned connecting element.